Electron microscopy study of the passivating layer on iron-nickel martensite

Abstract

A passivating oxide layer is known to form rapidly on the surface of Fe or Fe alloys exposed to air at low temperature. The properties of this passivating layer largely control the low temperature oxidation and corrosion properties of iron. It is hence important that the nature of this passivating layer be well understood. The work reported here principally involved a transmission electron microscopy examination of the thin oxide film formed on the surface of Fe−12Ni alloys on exposure to air at room temperature. Using high resolution microscopy the oxide particles of the film could be directly resolved and their structure and morphology characterized. The following conclusions were drawn from these characterization studies: 1) a passivating oxide layer (principally Fe3O4) forms spontaneously on the surface of Fe−Ni alloys on exposure to air at room temperature; 2) the orientation relation between the oxide and the metal surface depends on surface orientation; the Bain relation is obeyed when the alloy surface is (100)α while the Nishiyama-Wasserman relation is obeyed on other low index surfaces. This phenomenon is attributable to the need to accommodate the misfit strain between the oxide and the substrate; 3) the oxide layer consists of dispersed, small (∼20A) oriented particles rather than a continuous thin film; and 4) the size of the oxide particles and the density of their distribution is related to the crystal surface orientation and condition.